Imaging Techniques Used in Mesial Temporal Sclerosis

Introduction:

Mesial temporal sclerosis (MTS), also referred to as hippocampal sclerosis, is the most common temporal lobe epilepsy (TLE). Epilepsy or seizure is a disorder that occurs due to abnormal electrical activity in the brain cells, resulting in twitching, stiffness of muscles, and decreased awareness. Mesial temporal sclerosis is the abnormal hardening of tissues and the loss of neurons in the deeper part of the temporal lobe. The brain is divided into right and left sides, each with four lobes. The frontal lobe controls voluntary activities; the parietal lobe controls taste, temperature, touch, and movement; the occipital lobe controls vision; the temporal lobe contains memories and correlates them with taste, sight, sound, and touch sensations.

What Are the Symptoms of Mesial Temporal Sclerosis?

• Rising sensation in the stomach.

• Anxiety.

• Focal (partial) epilepsy.

• Strange sensations like fear, déjà vu (feeling of the familiarity of the situation), and Jamais vu (feeling of unfamiliarity).

• Behavioral and emotional changes.

• Muscle spasms.

• Generalized seizure.

• Unpleasant smell or taste.

• Loss of consciousness.

What Are the Different Types of Seizures Associated With Mesial Temporal Sclerosis?

1. Tonic-Clonic Seizure:

• A tonic-clonic seizure is also called a grand mal seizure. As per the name, this seizure has the characteristics of tonic and clonic seizures, in which tonic refers to stiffness, and clonic refers to rhythmic jerks. This seizure often lasts about one to three minutes, and sometimes, it may last more than five minutes, requiring immediate medical help. After the seizure, the person may be feeling irritable, sleepy, confused, and depressed. The tonic phase occurs first and is followed by the clonic phase.

• Tonic Phase: In this phase, all the muscles become stiff. The patient loses consciousness and suddenly falls to the floor. They may bite their inside cheeks or tongue, resulting in bloody saliva. Air forced past the vocal cord makes them groan or cry.

• Clonic Phase: In this phase, there are rapid and rhythmic jerks of the arms and legs, such as bending and relaxing the arms and legs at elbows, knees, and hips. The jerks slowly stop after a few minutes.

2. Focal Onset Impaired Awareness Seizure:

• This is the most common type of seizure that occurs in adults. This seizure begins from one side of the brain, which causes a change in their awareness level. This seizure was initially called a complex partial or psychomotor seizure and is sometimes referred to as a temporal lobe seizure if it arises from the temporal lobes.

• This seizure most often arises from the temporal or frontal lobe and tends to be longer than those from the frontal lobe. It starts with a focal awareness seizure or simple partial seizure in which patients can recall the events during a seizure and are alert and fully awake. This seizure usually lasts for about two minutes. Then, focal awareness seizures rapidly involve other areas and affect awareness and the ability to respond.

• Involuntary chewing movements, hand rubbing, and lip-smacking characterize this seizure. It usually lasts 30 seconds to three minutes.

What Are the Risk Factors for Mesial Temporal Sclerosis?

The risk factors for mesial temporal sclerosis are as follows:

• Family history.

• Autoimmune Disease - An abnormal condition in which the body's immune system attacks its healthy cells.

• Viral Infection - Certain viral infections, such as Herpesvirus type 6 and viral meningitis, can lead to inflammation in the temporal lobes and increase the risk of developing the condition.

• Encephalitis - Inflammation of the brain's tissues due to bacterial or viral infection.

• Febrile Seizure- A history of febrile seizures, such as prolonged or recurrent episodes during early childhood, is associated with an increased risk of developing the condition.

• Status Epilepticus - A medical emergency where a seizure occurs for more than five minutes or longer.

• Brain Injury- Brain injury due to trauma, accidents, sports injuries, infection, brain tumor (cancer), brain stroke, and uncontrolled seizure.

What Are the Imaging Techniques Used for the Diagnosis of Mesial Temporal Sclerosis?

Several imaging techniques are used to diagnose mesial temporal sclerosis to help visualize structural abnormalities, such as scarring in the brain's temporal lobes. The common imaging techniques are as follows:

1. Magnetic Resonance Imaging (MRI):

• In MRI, mesial temporal sclerosis appears as an abnormally increased T2 signal and decreased hippocampus volume. This increased signal is due to increased free water content and gliosis.

• Hippocampal sclerosis is the most common finding among patients with mesial temporal sclerosis.

• Abnormal morphology includes interdigitations of the hippocampus, a thin layer of white matter that divides the interlocking gyri, including the Ammon horn and dentate gyrus.

• Abnormal enlargement of the temporal horn of the lateral ventricle.

In Severe and Long Standing Cases:

The findings are as follows:

• Atrophy (decrease in size) of the mammillary (part of the hypothalamus) and ipsilateral fornix (part of the limbic system).

• Atrophy of the cingulate gyrus (part of the limbic system).

• Decreased volume of the subiculum (present in the mesial temporal lobe).

• Decreased volume and increased signal of the amygdala (part of the limbic system and mesial temporal lobe).

• Atrophy and increased signal of the anterior thalamic nucleus.

• Ipsilateral cerebral atrophy.

• Caudate and thalamic atrophy.

• Atrophy of the temporal lobe and dilatation of the temporal horn.

• Contralateral cerebellar hemiatrophy.

• Ipsilateral (affecting the same side) cerebral atrophy.

• Decreased volume of white matter in the parahippocampal gyrus (cortical ridge in the mesial temporal lobe).

2. MR Spectroscopy: MR spectroscopy shows neuronal abnormalities such as decreased myo-inositol (MI) in the ipsilateral temporal lobe and increased lactate and lipids immediately after a seizure. The N-acetyl aspartate (NAA) level decreases, as do the NAA/Cho and NAA/Cr ratios.

3. Diffusion MRI: It is an advanced MRI technique that measures the diffusion of water molecules in brain tissue. It allows the assessment of white matter integrity and connectivity and is used to detect abnormalities in the white matter tracts associated with mesial temporal sclerosis.

4. Nuclear Medicine: The two types of nuclear medicine include single photon emission computed tomography (SPECT) and positron emission tomography (PET). In the case of SPECT, a radioactive tracer, such as technetium-99m hexamethylpropylenemaine oxime, is administered. These are used to assess regional cerebral blood flow. They help identify regions of abnormal perfusion associated with MTS. PET imaging utilizes fluorodeoxyglucose to assess glucose metabolism in the brain. It helps confirm the presence of MTS.

What Are the Differential Diagnoses of Mesial Temporal Sclerosis?

• Absence Seizure - This seizure is most common in children and is characterized by staring at the space for a few seconds.

• Insular seizure arises from the insula (a brain structure) and manifests as unpleasant throat sensations and laryngeal discomfort.

• Occipital lobe seizure arises from the brain's occipital lobe and manifests as decreased vision, eye pain, and involuntary eye movement.

• Panic (fear) attacks.

• Excessive daytime sleepiness.

• Psychogenic Non-Epileptic Seizures (PNES) - PNES looks like epileptic seizures and not, and this arises from psychological events.

Conclusion

Magnetic resonance imaging (MRI) is precise (97 percent) and sensitive (80 to 90 percent) in the diagnosis of mesial temporal sclerosis. T2 weighted images of MRI can give detailed images of the internal architecture of the hippocampus in mesial temporal sclerosis than fluid-attenuated inversion recovery (FLAIR) images of MRI. Single-photon emission computed tomography (SPECT) and positron emission tomography (PET) scans are nuclear imaging techniques that use Tc-99m HMPAO and F18-FDG to diagnose mesial temporal sclerosis. Video EEG (electroencephalogram) monitoring helps differentiate mesial temporal sclerosis from other medical conditions, including tardive dyskinesia (a medical disorder that causes involuntary movements of the face, tongue, and extremities) and PNES.